In a plain-paper copier (PPC) or printer, toner images applied to the surface of paper or other recording medium are fixated by application of heat and pressure. In certain PPC machines fixation is accomplished by passing the image-bearing recording medium between a hot thermal-fixation roll and a pressure roll. When this type of thermal-fixation device is used the toner material is directly contacted by a roll surface and a portion of the toner adheres to the roll surface. With subsequent rotation of the roll the adhered toner material may be redeposited on the recording medium resulting in undesirable offset images, stains, or smears; or, in severe cases, the recording medium may stick to the adhered toner material on the roll and become wrapped around the roll.
To counter these problems materials having good release properties such as silicone rubber or polytetrafluoroethylene are often used for the roll surfaces. Although improving performance of the thermal fixation devices, use of silicone rubber or polytetrafluoroethylene roll surfaces alone do not eliminate the problems. Another approach used to counter the problems is to include release agents with the toner materials to prevent them from adhering to the roll surface. These oilless toners also improve performance of the thermal-fixation devices but again, particularly in the case of high-speed type copying machines, do not completely eliminate the problems associated with toner pickup and transfer.
Toner pickup by the rolls can be controlled by coating the surface of at least one of the rolls of a thermal fixation device with a liquid release agent, such as a silicone oil. It is important that the release liquid be applied uniformly and in precise quantities to the surface of the roll. It is also important that such be done in a manner to permit extended usage of the machine in order to minimize service costs and keep the cost per copy or printed page at a competitive level.
Means to supply release liquids to the heating and pressure rolls of a thermal fixation unit are known in the art and include wicks, pressure pads, and rolls. Such means usually include at least a thick porous material, such as felts of Nomex.RTM. fibers, glass fibers, carbon fibers, or polytetrafluoroethylene fibers, which may be covered with a porous permeation control material, such as porous polytetrafluoroethylene tubing or film. The thick porous material serves as a wick or reservoir for supplying the release liquid, usually a silicone oil, to the surface of a heating-, pressure-, or oil- transfer-roll. Also known in the art as means to supply controlled amounts of release liquids are oil-supply rolls having porous support cores of synthetic polymers, or elastomeric polymers, on the surface of which are porous permeation control layers formed of polytetrafluoroethylene film, or polytetrafluoroethylene film which has been impregnated with a mixture of silicone oil and silicone rubber followed by a heat treatment to crosslink the silicone rubber. Such rolls having highly compliant flexible surfaces are described in U.S. Pat. Nos. 5,123,151 (to Uehara, et al.), 5,232,499 (to Kato, et al.), and European Patent Application Publication No. 0 616 271 A2 (to Kikukawa, et al.).
Initially, thermal fixation units which incorporate such liquid supply devices perform satisfactorily and produce excellent high quality images. However, over a period of time, toner particles and agglomerates, paper particles, and other types of incidental dust and debris deposit on the heating and pressure rolls. The deposited debris can adversely affect the operation of a thermal fixation unit in a number of ways. Particles can damage the surface of the rolls by scratching, denting, or becoming embedded, and thus adversely influence image quality and fixation. Ultimately, they may also be transferred from the heating rolls, pressure rolls, or oil-transfer rolls to the surfaces of the release liquid supply devices, where they adversely influence uniformity and quantity of the oil supply, and where they may further damage the surface of contacting rolls.
To prevent, or at least minimize, damage from such particulate debris, cleaning mechanisms such as scraper blades, wiper blades, or separate cleaning rolls and brushes have been used. For the most part, these mechanisms have been applied so that the scraper blades, wiper blades, etc. are in direct contact with the surfaces of the heating rolls, pressure rolls, or oil-transfer rolls from which they are to remove excess toner, paper particles, and other debris. Damage to the roll surfaces by the scrapers and wipers can occur, as well as damage by particles trapped between the blades and the surfaces. Although such mechanisms significantly improve the length of service-free operation of thermal fixation units, none have prevented the eventual accumulation of particulate debris on the surfaces from which release oil is initially supplied and, consequently, the adverse effect on the uniformity and amount of oil supply which ensues from such accumulation.